The present invention relates to a power supply system or more in particular to a power factor improving system of a power supply system for rectifying and smoothing AC power from an AC power supply and applying DC power to a load.
In conventional systems for rectifying and smoothing an AC power supply and applying DC power to a load, an input current flows into a smoothing circuit in pulse form from the AC power supply. This has disadvantages of a lower power factor of the power supply and a large effective value of the input current.
A system has been previously suggested in which in order to improve the power factor of a power supply a reactor is inserted in series between an AC power supply and a rectifier circuit, a switching circuit is connected in parallel with the rectifier circuit as the input side thereof. In this arrangement, the switching circuit is operated only during the time when the input current is not applied to the rectifying circuit thereby to store the energy in the reactor intermittently. Thus, the energy stored in the reactor is discharged via the rectifier circuit to a capacitor making up a smoothing circuit so that the input current is applied intermittently thereby to improve the distortion of the current waveform with respect to the voltage waveform, thus improving the power factor of the power supply. (Japanese Patent Laid-Open Application No. 17931/78).
In this conventional system comprising a power factor improving circuit connected in parallel to the input terminal of the rectifier circuit and a switching element including a transistor for intermittently shorting the rectifier circuit, however, the current in the same direction is supplied to the transistor of the power factor improving circuit, thus requiring at least four diodes as in the rectifier circuit. Further, the control circuit for supplying a drive signal for driving the transistor must be isolated from the power circuit by some means.
Furthermore, in a system wherein an AC motor is driven through an inverter as a load, for instance, the AC current is required to be detected to actuate the power factor improving circuit effectively in accordance with the output condition of the inverter, and this AC current detector element must also be insulated by some means.
What is more, in view of the fact that the time TA of the switching operation and the duty cycle DC of the switching element are normally set to attain the maximum power factor Pf at the rated state of the load and are constant regardless of the load condition, the power factor Pf is very low depending on the magnitude of the load especially under a low load.